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III. Environmental Setting, Impacts, and Mitigations H. NOISE SETTING Introduction Ambient noise is the total noise associated with a given environment,and usually includes all sources both near and far. Environmental noise is usually measured in A-weighted decibels (dBA),I Some representative noise sources and their corresponding noise levels are shown in Figure III.H.1. Environmental noise levels typically fluctuate over time,and different types of noise descriptors are used to account for this variability. Typical noise descriptors,used to describe time-averaged noise levels,include the energy-equivalent noise level (Leq),the day- night average noise level (Ldn), and the community noise equivalent level (CNEL).2 Both the Ldn and CNEL noise descriptors are commonly used in establishing noise exposure guidelines for specific land uses. Noise levels are measured on a logarithmic scale, and therefore,are not added in the usual arithmetic manner(Cuniff, 1977). A doubling of sound energy results in a three-dBA increase in noise levels,which is the smallest change in noise level detectable by the average person. A noise level increase of ten dBA is perceived as being twice as loud. Generally,in areas where existing noise levels are dominated by traffic,a doubling in the volume of vehicular traffic will cause ambient noise levels to increase by three dBA. The noise level experienced at a receptor depends on the distance between the source and the receptor,presence or absence of noise barriers and other shielding devices,and the amount of noise attenuation (lessening)provided by the intervening terrain. For line sources, such as motor I A decibel(dB)is a logarithmic unit of sound energy intensity. Sound waves,traveling outward from a source,exert a sound pressure level(commonly called"sound level")measured in decibels. An A- weighted decibel(dBA)is a decibel corrected for the variation in frequency response of the typical human ear at commonly encountered noise levels. 2 Leq,the energy equivalent noise level,is the equivalent steady-state continuous noise level which,in a stated period of time,would contain the same acoustic energy as the time-varying sound level actually measured during the same period. Ldn,the day-night average noise level,is a calculated noise descriptor based on average hourly noise levels(Leq)over a 24-hour period. Noise between 10:00 p.m.and 7:00 a.m.is weighted by adding 10 dBA to take into account the greater annoyance of nighttime noise. CNEL,the community noise equivalent level,is similar to Ldn,but an additional five dBA"penalty"is added to evening noise(7:00 p.m.to 10:00 p.m.). Ldn and CNEL are considered equivalent for most planning purposes. III.H.1